Grain oriented electrical steel sheet having superior magnetic properties, and manufacturing process thereof

ABSTRACT

A grain oriented electrical steel having superior magnetic properties and for use in transformers and electrical generators, and a manufacturing process thereof are disclosed. Cu and P are mixedly added in the melting stage of a silicon steel containing MnS and AlN as grain growth inhibitors, and in this way, the magnetic properties are improved. The chemical composition of the steel sheet of the present invention are: 2.50-4.00% of Si, 0.03-0.15% of Mn, 0.030-0.300% of Cu, and 0.020-0.200% of P, the balance being Fe, all in weight %. The grain oriented electrical steel sheet of the present invention shows a low iron loss and a high magnetic flux density, and is cold-rolled to a thickness range of 0.15-0.27 mm.

FIELD OF THE INVENTION

The present invention relates to a grain oriented electrical steel sheetused as steel cores for transformers and electrical generators and amanufacturing process thereof, and particularly to a grain orientedelectrical steel sheet and a manufacturing process thereof, in which thesteel sheet has superior magnetic properties such as low iron loss andhigh magnetic flux density, as well as being applicable to thin gaugeproducts.

BACKGROUND OF THE INVENTION

Generally, the grain oriented electrical steel is a soft magneticmaterial exhibiting superior magnetic properties in its rollingdirection, and this material has to be easy to magnetically excite andlow in its iron loss. The exciting property is evaluated based on thelevel of the magnetic flux density B₁₀ which is induced by a certainlevel of magnetizing force (1000 A/m), while the iron loss is evaluatedby the magnitude of energy loss (W_(17/50)) which occurs when the steelis induced to a certain level of magnetic flux density (1.7 Tesla) by analternating current of a certain frequency (50 Hz).

A material showing a high magnetic flux density is usually used inminiature high performance electrical apparatuses, while a low iron lossmeans a low energy dissipation.

In a grain oriented electrical steel sheet which consists of crystalgrains having an orientation of (110) [001] in the Miller indices, ifthe magnetic flux density and the iron loss properties are to beimproved, the orientation of the steel has to be improved. That is, thedirection [001], which is the direction of easy magnetization, has tocorrespond with the rolling direction of the steel sheet.

The grain oriented electrical steel in the industrial field ismanufactured by utilizing the so-called secondary recrystallizationphenomenon which occurs during the final annealing process (which iscarried out at a high temperature of over 1000° C.,) after cold-rollingthe steel sheet to the final thickness, and after subjecting it to adecarburizing annealing.

During the secondary recrystallization, the grains having theorientation of (110)[001] devour surrounding grains having the otherorientation and grow to very large sized grains.

If such a secondary recrystallization is to be producted in a perfectmanner, there is required an inhibiting force which inhibits the normalgrowth of the primary recrystallization grains of the otherorientations, during the growth of the secondary recrystallizationgrains.

Further, recently in pace with the increased need for the saving ofenergy, it is demanded that the thickness of the steel sheet be reducedin addition to the improvement of orientation in order to improve theiron loss. This is due to the fact that eddy current loss which occupiesthe greater part of the iron loss is proportionate to a square of thethickness of the steel sheet, and that the thinner the thickness ofsteel sheet is, the smaller the iron loss is. However, if the thicknessof the steel sheet is made thinner, not only the secondaryrecrystallization becomes unstable, but also the orientation isdegraded. Therefore, the lower limit of the thickness of the grainoriented electrical steel sheet which can be manufactured in a stablemanner by the normal method is about 0.30 mm.

Therefore, if the iron loss is to be improved by reducing the thicknessof the steel sheet, the inhibiting force against the normal growth hasto be reinforced, so that the secondary recrystallization should occurin a perfect manner.

As a method of inhibiting grain growth during the manufacturing of thegrain oriented electrical steel sheet, it is known that one or more ofprecipitating compounds such as MnS, AlN, MnSe and the like or grainboundary segregating elements are added at the melting stage, and that aprecipitation treatment is carried out on the steel sheet at a laterstep of the process.

According to Zener's formula, the inhibiting force is defined to beσΩ/Υο (Υο: average particle size of the precipitates, Ω: volume fractionof the precipitations, and σ: grain boundary energy). According to thisformula, if the value of Υο is small, and if Ω is large, then theinhibiting force is increased. That is, if fine precipitates can beformed, a sufficient inhibiting force can be obtained with only theprecipitations, the logical conclusion being so. However, in actuality,there is a limit to simultaneously achieving a large amount ofprecipitates and a reduction of the their size, and therefore, it shouldbe effective to add and distribute two or more precipitating compoundsor grain boundary segregating elements.

In the method for improving the orientation of the grain orientedelectrical steel as described above, if a high reduction ratio is usedin the final cold rolling process, the driving force for the growth ofthe primary recrystallization grains is increased, and therefore, alarger inhibiting force is required.

For example, a magnetic flux density of about 1.8 Tesla is obtained bycarrying out a cold rolling process using a reduction ratio of 60% inone of the conventional oriented electrical steels. In such a case, MnSprecipitates are used as main inhibitors. On the other hand, in anotheroriented electrical steel in which a magnetic flux density of 1.90 Teslais obtained by carrying out a cold rolling process using a higherreduction ratio of over 80%, two or more of precipitating compounds suchas MnS and AlN are used as the inhibiting agents.

Further, according to Japanese Patent Publication No. Sho57-45818, thegrain growth inhibiting force is reinforced by adding Cu as a sulfideforming element in addition to MnS and AlN, and a reduction ratio of 87%is applied, thereby providing a process for manufacturing a grainoriented electrical steel sheet having superior magnetic properties.

Meanwhile a process of adding P in the melting stage of the grainoriented electrical steel is disclosed in Japanese Patent PublicationNo. Sho-52-6329. By adding P, the precipitates such as MnS and AlN canbe more uniformly distributed in the form of tiny particles, andconsequently, the secondary recrystallization grains become more fine,thereby improving the iron loss properties. However, if the effect ofthe addition of P is to be obtained, Ni has to be necessarily added,and, if its addition is less than 0.03%, the secondary recrystallizationbecomes unstable.

SUMMARY OF THE INVENTION

Therefore it is the object of the present invention to provide a grainoriented electrical steel sheet and a manufacturing process thereof, inwhich the secondary recrystallization grains can be developed in astable manner with an acceptable orientation even with a thin thickness,thereby providing a high magnetic flux density and low iron lossoriented electrical steel sheet.

The present inventors have performed repeated experiments in order tofind a process for manufacturing a high magnetic flux density and lowiron loss oriented thin electrical steel sheet by adding elementscontributing to reinforcing the inhibiting force. The present inventorstried the following process. That is, Cu and P were added in the amountsof 0.030-0.300% and 0.020-0.200% respectively in the melting stage of asilicon steel containing MnS and AlN as the basic inhibiting agents, andthen, the normal manufacturing process which is usually carried out onthe high magnetic flux density oriented electrical steel sheet wasperformed. For such a case, the present inventors found that, even whenthe thickness of the cold rolled steel sheet was 0.15-0.27 mm as wellfor the case of 0.30-0.35 mm, a good oriented secondaryrecrystallization was developed in a stable manner, thereby making itpossible to obtain a low iron loss and high magnetic flux densityoriented electrical steel sheet.

An electron micrograph showed that Cu which is added at the meltingstage forms precipitates in the form of Cu₂ S, and P is segregated onthe grain boundary. Based on this fact, it can be asserted that, if Cuand P are added into a silicon steel containing MnS and AlN, the graingrowth inhibiting force is more reinforced, with the result that thesecondary recrystallization is developed in a stable manner, and thatits orientation is more improved.

Based on the above described facts, the present invention provides a lowiron loss and high magnetic flux density oriented electrical steel sheetand a manufacturing process thereof, in which the grain growthinhibiting force is reinforced by mixedly adding Cu and P at the meltingstage, thereby forming a grain oriented electrical steel sheet which canbe applied even to thin gauge products.

BRIEF DESCRIPTION OF THE DRAWINGS

The above object and other advantages of the present invention willbecome more apparent by describing in detail the preferred embodiment ofthe present invention with reference to the attached drawing in which:

FIG. 1 is a graphical illustration showing the variation of thesecondary recrystallization versus the addition ratio of Cu and P(Cu/P).

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a grain oriented electrical steel sheethaving superior magnetic properties, in which the chemical compositionis Si: 2.50-4.00%, Mn: 0.03-0.150%, Cu: 0.030-0.300%, P: 0.020-0.200%,Fe: balance,. all in weight %.

More specifically, the grain oriented electrical steel sheet of thepresent invention is manufactured in the following manner. That is, Cuand P are added in the amounts of 0.030-0.300% and 0.020-0.200%respectively in the melting stage of a silicon steel which contains:0.0300-0.100% of C, 2.50-4.00% of Si, 0.030-0.150% of Mn, 0.010-0.050%of S, 0.010-0.050% of soluble Al, and 0.0030-0.0120% of N, the balancebeing Fe, all in weight %. Then the silicon steel slab undergoes furtherprocessing such as hot rolling, precipitation annealing, acid washing,cold rolling, decarburizing annealing, coating of an annealing separatorand high temperature annealing, thereby obtaining a grain orientedelectrical steel sheet having superior magnetic properties.

Now the reasons for limiting the values of the additions will bedescribed below.

If the added amount of C is less than 0.030 weight % (to be expressed"%" below), the crystallized grains in the slab are coarsely grown, withthe result that the development of the secondary recrystallizationbecomes unstable during the final high temperature annealing, therebymaking it undesirable. On the other hand, if it exceeds 0.100%, too muchtime is required for carrying out the decarburizing annealing process,thereby making it also undesirable.

If Si is added in an amount less than 2.50%, a low iron loss propertycannot be obtained, while, if it exceeds 4.00%, the cold rollability isdegraded.

Mn and S are the elements which are needed for forming precipitations,and, of them, if Mn is added in an amount departing from the range of0.030-0.150%, a proper distribution of MnS for inhibiting the graingrowth cannot be achieved. Meanwhile, if the addition of S exceeds0.050%, the de-sulphurizing cannot be carried out sufficiently duringthe final high temperature annealing degration in the magneticproperties, while if it is added in an amount less than 0.010%, asufficient amount of precipitation in the form of a sulfide cannot beobtained, thereby making it undesirable.

The soluble Al and N are the elements which are needed for formingprecipitates, and, of them, if the soluble Al is added in an amount lessthan 0.010%, the orientation of the secondary recrystallization isdeteriorated causing the magnetic flux density to be lowered, while, ifit exceeds 0.050%, the development of the secondary recrystallizationbecomes unstable, thereby making it undesirable. Therefore a moredesirable range of the addition of the soluble Al is 0.020-0.030%.Meanwhile, if N is added in an amount less than 0.0030%, the amount ofAlN becomes insufficient, while, if it exceeds 0.0120%, a defect in theform of blisters is produced in the final products.

As for Cu and P which are the characteristic feature of the presentinvention, their most effective addition ranges are 0.030-0.300% for Cu,and 0.020-0.200% for P. If the stability of the development of thesecondary recrystallization and the improvement of the orientation ofthe secondary recrystallization are considered, their most effectiveaddition ranges are 0.050-0.150% for Cu and 0.040-0.120% for P. Cu isthe element which is needed for forming Cu₂ S, and, if it is added in anamount less than 0.030%, a sufficient amount of precipitates in the formof Cu₂ S cannot be obtained, so that, if it is manufactured in athickness thinner than the normal one, the secondary recrystallizationcannot be formed in a stable manner. Meanwhile, if it exceeds 0.300%,although the secondary recrystallization can be formed, its orientationis aggravated. Meanwhile, P is a grain boundary. segregating elementwhich improves the grain growth inhibiting force, and, if this elementis added in an amount less than 0.020%, superior magnetic propertiescannot be obtained, while, if it exceeds 0.200%, the cold rollability isdeteriorated.

Within the above addition ranges of Cu and P, the addition ratio of them(Cu/P) should be most desirably 0.50-3.00, because, if the value of Cu/Pis less than 0.50, the formation rate of the secondary recrystallizationgrains are lowered to some degree, while, if the value of Cu/P exceeds3.00, the magnetic flux density, i.e., the orientation of the secondaryrecrystallization, tends to be aggravated.

By applying a melting process, and an ingot making process (orcontinuous casting process) in the normal manner, the silicon steelwhich is manufactured in the above described manner is made suitable forcarrying out the succeeding processes which are usually performed forthe normal high magnetic flux density oriented electrical steel sheet.

The silicon steel having the chemical composition as described above isused as a material for manufacturing a high magnetic flux densityoriented electrical steel sheet, and the process for manufacturing sucha steel sheet will be described below.

The silicon steel slab of the present invention is rolled to a certainthickness by applying the normal hot rolling process. The hot rolledplate undergoes a precipitation annealing at a temperature of 950-1200°C. for 30 seconds-30 minutes in order to adjust the precipitating stateof AlN, and then, is subjected to a quenching process. This plate whichhas undergone the precipitation annealing process is subjected to apickling process, and then is subjected to one round of cold rolling, oris subjected to two or more rounds of cold rolling processes includingan intermediate annealing process.

The final cold rolling reduction ratio (the relevant reduction ratio isused for the case of performing only one round of cold rolling) may beas high as 65-95%, or more desirably as high as 80-92%. The reductionratios for other than the last rolling process are not important, andtherefore, they will not be defined here. Between the plural passes ofthe cold rolling processes, aging processes are performed at atemperature of 100-300° C. for 30 seconds-30 minutes in order to improvethe magnetic properties.

In carrying out the cold rolling, the sheet may be cold-rolled to afinal thickness of 0.27-0.35 mm, the magnetic properties being superiorin this case. However, more desirably, the sheet can be cold-rolled to athickness range of 0.15-0.27 in order to further reduce the iron loss.The reason for the desirableness of the above range is that, if thefinal thickness is less than 0.15 mm, the secondary recrystallizationdoes not develop in a stable manner. On the other hand, if it is over0.27 mm, the reduction of the iron loss due to the reduction of thethickness becomes meager, although the secondary recrystalization occursin a stable manner.

The steel sheet which is cold rolled in the above described manner isdecarburized and primarily recrystallized by being subjected to adecarburizing annealing process. In the present invention, thedecarburizing annealing process is desirably carried out at atemperature of 800-900° C. for 30 seconds-10 minutes under an atmosphereof humid hydrogen or a mixed atmosphere of humid hydrogen and nitrogen.After carrying out the decarburizing annealing process, an annealingseparator is coated on the surfaces of sheets in order to preventsurface to surface adherence and to promote the formation of glassfilms.

As the annealing separator, MgO, TiO₂ and Na₂ B₄ O₇ may be used as themajor ingredients. Then this sheet is subjected to a high temperatureannealing process at a temperature of 1200° C. for over 5 hours for thesecondary recrystallization and for a purification, and, as theatmosphere for this process, dry pure hydrogen or a mixture of hydrogenand nitrogen may be used. After carrying out this annealing, aninorganic glass film is formed on the surface of the steel sheet, but itis desirable to perform a coating in order to give a tension for thepurpose of improving the iron loss through the reduction of the size ofthe magnetic domains.

The grain oriented electrical steel sheet manufactured in the abovedescribed manner has the following chemical composition: 2.50-4.00% ofSi; 0.030-0.150% of Mn; 0.030-0.300% of Cu; and 0.020-0.200% of P, thebalance being Fe. Here, Si is an element which increases the inherentresistivity of the steel sheet to provide a low iron loss, while Mn, Cuand P are needed to promote the development of the secondaryrecrystallization grains having a nice orientation. The other elementssuch as C, S, N and Al are indispensable in developing the secondaryrecrystallization, but these elements are almost removed during thedecarburizing annealing process and the final high temperature annealingprocess, and they remain only in negligible amounts in the finalproducts. Meanwhile, the other elements such as Si, Mn, Cu and P remainin the steel sheet intact even after undergoing the decarburizingannealing process and the final high temperature annealing process, butthey do not deteriorate magnetic properties. Therefore, the reason forlimiting the amounts of the elements such as Si, Mn, Cu and P is same asthe reason for limiting their amounts during the manufacturing process.

Now the present invention will be described based on actual examples.

Example 1

As shown in Table 1 below, a silicon steel slab (thickness: 40 mm)containing C, Si, Mn, S, soluble Al and N was prepared, and anothersilicon steel slab (thickness: 40 mm) containing Cu and P in addition tothe above elements was prepared. These silicon steel slabs were heatedto a temperature of 1350° C., and then, were hot-rolled to a thicknessof 2.3 mm. Then they were annealed at a temperature of 1200° C. for 4minutes, then were slowly cooled down to a temperature of 1200° C., andthen, were quenched in a boiling water of 100° C.

Thereafter, a pickling process was carried out, and then, cold rollingprocesses were carried out to obtain a final thickness of 0.20 mm.

Between the passes of the cold rolling processes, aging treatments werecarried out at a temperature of 200° C. for 5 minutes, and decarburizingannealing processes were carried out at a temperature of 840° C. underan atmosphere of a gas mixture of hydrogen (75%) and nitrogen (25%) for3 minutes.

Then an annealing separator containing ingredients of MgO, TiO₂ and Na₂B₄ O₇ was coated on the sheets. Then a final high temperature annealingprocess was carried out at a temperature of 1200° C. for 20 hours, andthen, a tension coating fluid containing major ingredients of aluminumphosphate, anhydrous chromate and colloidal silica was coated. Then anannealing process was performed at a temperature of 840° C. for oneminute for flattening the steel sheets, and then, the secondaryrecrystallization development rate and the magnetic properties weremeasured, the measured results being as shown in Table 1 below.Meanwhile, the Chemical compositions of the respective steels are shownin Table 2 below.

                                      TABLE 1                                     __________________________________________________________________________                           secondary                                                  chemical composition                                                                             recrystallization                                                                      Magnetic                                      Steel                                                                             (weight %)         rate (%) properties                                    sheets                                                                            C  Si Mn S  Al*                                                                              N   Cu P     BlO                                                                              W17/50                                     __________________________________________________________________________    Com A                                                                             0.083                                                                            3.14                                                                             0.077                                                                            0.027                                                                            0.027                                                                            0.0080                                                                            -- -- 70 1.72                                                                             1.39                                       Com B                                                                             0.082                                                                            3.14                                                                             0.079                                                                            0.028                                                                            0.028                                                                            0.0070                                                                            0.010                                                                            -- 95 1.80                                                                             1.30                                       Com C                                                                             0.083                                                                            3.15                                                                             0.076                                                                            0.026                                                                            0.028                                                                            0.0078                                                                            -- 0.080                                                                            80 1.83                                                                             1.26                                       Invt                                                                              0.082                                                                            3.15                                                                             0.077                                                                            0.027                                                                            0.026                                                                            0.0079                                                                            0.090                                                                            0.060                                                                            100                                                                              1.93                                                                             1.09                                       Com D                                                                             0.032                                                                            3.14                                                                             0.078                                                                            0.028                                                                            0.028                                                                            0.0079                                                                            0.520                                                                            0.075                                                                            100                                                                              1.87                                                                             1.10                                       Com E                                                                             0.083                                                                            3.14                                                                             0.076                                                                            0.027                                                                            0.027                                                                            0.0078                                                                            0.070                                                                            0.230                                                                            Destroyed***                                     __________________________________________________________________________

                  TABLE 2                                                         ______________________________________                                               Chemical composition                                                   Steel  (weight %)                                                             sheets Si     Mn     Cu    P    Other elements                                ______________________________________                                        Com A  3.14   0.077  --    --   Fe and tiny amounts of                                                        Al, C, N, S                                   Com B  3.14   0.077  0.098 --   Same                                          Com C  3.15   0.076  --    0.079                                                                              same                                          Invt   3.15   0.076  0.090 0.059                                                                              Same                                          Com D  3.14   0.077  0.316 0.075                                                                              Same                                          ______________________________________                                    

In the above tables, "Com" indicates comparative steel sheets, and"Invt" indicates the steel sheets of the present invention, while othersymbols are as follows.

*: amount of soluble aluminum.

**: The unit of magnetic flux density (B₁₀) is Tesla, and the unit ofiron loss (W_(17/50)) is W/kg.

***: During the cold rolling process, the steel sheets were severelydamaged, and therefore, the subsequent processes could not be performed.

In the above table, the secondary recrystallization rate (%) wasmeasured in such a manner that the steel sheet was etched with a boilingchloric acid after carrying out the final high temperature annealingprocess, and then the macro structure was observed, thereby deciding thearea ratio occupied by the secondary recrystallized grains. In otheractual examples to be described below, the measurements are carried outin the same manner.

As shown in Table 1 above, when a cold rolling was carried out to athickness of 0.20 mm, the comparative sheet A containing only MnS andAlN showed unstable developments of secondary recrystallization grains,thereby deteriorating the magnetic properties. When only Cu was added(as in the case of the comparative steel sheet B), although thesecondary recrystallization was developed in an acceptable manner, themagnetic flux density was drastically lowered, thereby making itimpossible to obtain superior magnetic properties. Further, among thesteel sheets containing precipitates such as MnS and AlN, if only P wasadded (as in the case of the comparative steel sheet C), the secondaryrecrystallization development rate was very low, thereby making thesheet unsuitable for cold-rolling to a thin thickness.

On the other hand, in the case of the steel sheet of the presentinvention in which proper amounts of Cu and P were mixedly added, thesecondary recrystallization was developed in a perfect manner even undera thin thickness, as well as giving superior orientation by making itpossible to induce a high magnetic flux density. However, even if Cu andP were mixedly added, if the amount of Cu exceeded 0.300% (as in thecase of the comparative steel sheet D), an acceptable magnetic fluxdensity could not be obtained, although the secondary recrystallizationwas developed in a perfect manner. Meanwhile, if the amount of Pexceeded 0.200% (as in the case of the comparative steel sheet E), thesteel sheet was severely damaged during the cold rolling process,thereby making it impossible to measure magnetic properties.

Meanwhile, among the elements which are contained in the silicon steelsheet (as shown in Table 1), Al, C, N and S were almost removed duringthe annealing processes, leaving only tiny amounts of them. However, theother elements such as Si, Mn, Cu and P remained intact in the finalproducts, as shown in Table 2 above.

Example 2

A silicon steel slab was prepared, the slab containing 0.073% of C,3.13% of Si, 0.075% of Mn, 0.027% of S, 0.026% of soluble Al, and0.0073% of N, and another same silicon steel slab was prepared in which0.080% of Cu and 0.080% of P were added in the melting stage. Theseslabs were subjected to hot rolling processes in the normal manner toreduce them to a thickness of 2.3 mm. Then they were annealed at atemperature of 1130° C. for 1 minute, were slowly cooled down to 930°C., and then, were quenched in a boiling water of 100° C. Then acidwashes were carried out, and then, cold rolling processes were carriedout, thereby obtaining cold rolled steel sheets having thicknesses of0.35, 0.30, 0.27, 0.23, 0.20, 0.18, 0.15 and 0.12 mm. Between the passesof the cold rolling process, aging treatments were carried out at atemperature of 180° C. for 5 minutes. Then a decarburizing annealingprocess was carried out at a temperature of 830° C. for 5 minutes underan atmosphere of a gas mixture of nitrogen (75%) and hydrogen (25%)having a dew point of 55° C. Then an annealing separator containingmajor ingredients of MgO, TiO.sub. 2 and Na₂ B₄ O₇ was coated. Then afinal high temperature annealing was carried out at a temperature of1200° C. for 20 hours. Thereafter, a tension coating fluid containingmajor ingredients of aluminum phosphate, anhydrous chromic acid, andcolloidal silica was coated, and then, a flattening annealing processwas carried out at a temperature of 850° C. for one minute. Thenmeasurements were carried out on the variations of the magneticproperties and the secondary recrystallization rates as against thevariations of the final sheet thickness, and the measured results areshown in Table 3 below.

                  TABLE 3                                                         ______________________________________                                        Cold                                                                          rolled Secondary Magnetic prpts                                               thickness                                                                            recstllzn B.sub.10                                                                              W.sub.17/50                                          (mm)   (%)       (Tesla) (W/kg) Additions                                                                             Remarks                               ______________________________________                                        0.35   100       1.94    1.19   MnS, AlN,                                                                             Invt 1                                                                P                                             "      100       1.93    1.21   MnS, AlN                                                                              Com a                                 0.30   100       1.94    1.21   MnS, AlN,                                                                             Invt 2                                                                Cu, P                                         "      100       1.92    1.17   MnS, AlN                                                                              Com b                                 0.27   100       1.94    1.07   MnS, AlN,                                                                             Invt 3                                                                Cu, P                                         "       94       1.88    1.24   MnS, AlN                                                                              Com c                                 0.23   100       1.93    1.04   MnS, AlN,                                                                             Invt 4                                                                Cu, P                                         "       85       1.81    1.35   MnS, AlN                                                                              Com d                                 0.20   100       1.94    1.01   MnS, AlN,                                                                             Invt 5                                                                Cu, P                                         0.18    98       1.9     0.99   MnS, AlN                                                                              Invt 6                                                                Cu, P                                         0.15    95       1.91    0.98   MnS, AlN                                                                              Invt 7                                                                Cu, P                                         0.12    80       1.78    1.34   MnS, AlN                                                                              Com e                                                                 Cu, P                                         ______________________________________                                    

In the above table, "Com" indicates the comparative steel sheets, and"Invt" indicates the steel sheets of the present invention.

As shown in Table 3 above, the steel sheets (1-7) of the presentinvention, in which proper amounts of Cu and P are added in addition toMnS and AlN, show superior magnetic properties over the comparativesteel sheets (a-d) containing only MnS and AlN, for the same cold rolledthickness. Further, even with the thin thicknesses of 0.15-0.27 mm, thesteel sheets (3-7) of the present invention show stable development ofsecondary recrystallizations, and also show high magnetic flux densitiesand low iron losses. Meanwhile, the comparative steel sheet (e) whichhas a thickness of 0.12 mm shows a low magnetic flux density and a highiron loss, although it comes within the same composition range as thatof the present invention.

Example 3

To silicon steel slabs containing 0.073% of C, 3.12% of Si, 0.070% ofMn, 0.025% of S, 0.024% of soluble Al, 0.0071% of N and 0.11% of Cu, theelement P was added in three different amounts within the addition rangeof the present invention, i.e., in the amounts of (A) 0.020%, (B) 0.070%and (C) 0.200%.

These slabs were subjected to hot rolling processes to reduce them to athickness of 2.3 mm, and then subjected to a first cold rolling processto reduce them to a thickness of 1.57 mm, after carrying out a picklingprocess. Then they were annealed at a temperature of 1100° C. for 3minutes, slowly cooled down to a temperature of 950° C., and then, werequenched in a boiling water of 100° C. Then a pickling process wasperformed again, and then, a second cold rolling was performed to reducethem to a thickness of 0.23 mm. Between the passes of the cold rollingprocess, aging processes were carried out at a temperature of about 150°C. for 10 minutes. Then a decarburizing annealing was carried out at atemperature of 850° C. for 90 seconds under an atmosphere consisting ofa gas mixture of nitrogen (25%) and hydrogen (75%) having a dew point of65° C. Then an annealing separator having ingredients of MgO, TiO₂ andNa₂ B₄ O₇ was coated, and thereafter, a final high temperature annealingwas carried out at a temperature of 1180° C. for 20 hours. Then atension coating fluid containing major ingredients of aluminumphosphate, anhydrous chromic acid and colloidal silica was coated, andthen, a flattening annealing was carried out at a temperature of 800° C.for 1.5 minutes. After completing the whole process, the secondaryrecrystallization development rate (%) and the magnetic properties weremeasured, the measured results being as shown in Table 4 below.

                  TABLE 4                                                         ______________________________________                                                secondary   Magnetic properties                                                 recstllzn     B.sub.10                                                                              W.sub.17/50                                   Steel sheets                                                                            rate (%)      (Telsa) (W/Kg)                                        ______________________________________                                        A         98            1.92    1.06                                          B         100           1.95    1.03                                          C         95            1.93    1.05                                          ______________________________________                                    

As shown in Table 4 above, if P is added in an amount within theaddition range of the present invention, the secondary recrystallizationoccurred in a stable manner and the magnetic properties were alsosuperior, when the cold rolled thickness was 0.23 mm. However, with thevalue of Cu/P being adjusted to 1.57 as in the case of the steel sheetB, the magnetic properties were further improved.

Example 4

To silicon steel slabs containing 0.079% of C, 3.15% of Si, 0.073% ofMn, 0.029% of S, 0.028% of soluble Al, 0.0082% of N and 0.055% of P, theelement Cu was added in three different amounts within the additionrange of the present invention, i.e., in the amounts of (D) 0.030%, (E)0.080% and (F) 0.300%. These slabs were hot-rolled to a thickness of 2.0mm in the normal manner, and then, were annealed at a temperature of1120° C. for two minutes. Then they were slowly cooled down to atemperature of 950° C., and were quenched in a boiling water of 100° C.Then a pickling process was carried out, and then, a cold rolling wascarried out to reduce them to a final thickness of 0.18 mm. Between thepasses of the cold rolling process, aging treatments were carried out ata temperature of 200° C. for 5 minutes. Then a decarburizing annealingwas carried out at a temperature of 850° C. for 90 seconds under anatmosphere consisting of a gas mixture of nitrogen (25%) and hydrogen(75%) having a dew point of 68° C. Then an annealing separatorcontaining a mixture of MgO, TiO₂ and Na₂ B₄ O₇ was coated, and then, afinal high temperature annealing was carried out at a temperature of1180° C. 20 hours. Then a tension coating fluid containing majoringredients of aluminum phosphate, anhydrous chromic acid and colloidalsilica was coated, and then, a flattening annealing was carried out at atemperature of 850° C. for 50 seconds. After completing the wholeprocess, the secondary recrystallization rate and the magneticproperties were measured, the measured results being as shown in Table 5blow.

                  TABLE 5                                                         ______________________________________                                                secondary   Magnetic properties                                                 recstllzn     B.sub.10                                                                              W.sub.17/50                                   Steel sheets                                                                            rate (%)      (Telsa) (W/Kg)                                        ______________________________________                                        D         95            1.93    1.03                                          E         100           1.94    1.01                                          F         93            1.90    1.05                                          ______________________________________                                    

As is apparent in Table 5 above, if the amount of Cu is varied withinthe addition range of the present invention, the secondaryrecrystallization occurred in a stable manner, and superior magneticproperties were also obtained, even with a cold rolled thickness of 0.18mm. The steel sheet E in which the value of Cu/P is 1.46 shows the mostadvantageous iron loss characteristics.

Example 5

Cu and P were mixedly added at a melting stage into silicon steelscontaining 0.077% of C, 3.17% of Si, 0.076% of Mn, 0.028% of S, 0.025%of soluble Al and 0.0075% of N, the balance being Fe. The addition ratio(Cu/P) was varied within the range of 0.25-6.50 when preparing thesilicon steel slabs having a thickness of 40 mm. The subsequent steps ofthe process were the same as that of Example 1, except that the finalthickness of the steel sheet was 0.23 mm. After completing the wholeprocess, the rate and orientation of the secondary recrystallizationwere measured, the measured results being as shown in FIG. 1.

Referring to FIG. 1, the orientation of the secondary recrystallizationis expressed in the value of magnetic flux density B₁₀. As can be seenin FIG. 1, when an electrical steel sheet is manufactured to a thicknessof 0.23 mm by mixedly adding Cu and P, if the value of Cu/P comes withinthe range of 0.50-3.00, then it is seen that the secondaryrecrystallization rate and the magnetic properties B₁₀ are superior.However, if the value of Cu/P is less than 0.50, the secondaryrecrystallization rate is lowered, while if it is over 3.0, the magneticflux density B₁₀, i.e., the orientation of the secondaryrecrystallization is deteriorated.

According to the present invention as described above, Cu and P aremixedly added at a melting stage of a silicon steel containing MnS andAlN as the grain growth inhibitors, and the silicon steels are finallycold-rolled to a thickness of 0.15-0.27 mm, thereby producing a highmagnetic flux density and low iron loss oriented electrical steel sheetswhich are applicable even to thin gauge products.

What is claimed is:
 1. A grain oriented electrical steel sheet havingsuperior magnetic properties on the order of greater than about 1.9Tesla magnetic flux density (B₁₀) and less than about 1.1 W/Kg iron orcore loss (W_(17/50)), said steel sheet in a decarburized condition,having a thickness within the range of about 0.15-0.27 mm, andconsisting essentially of:2.50-4.00% of Si, 0.030-0.150% of Mn,0.030-0.300% of Cu and 0.020-0.200% of P all in weight %, the balancebeing Fe and negligible amounts of C, S, N and Al.
 2. The grain orientedelectrical steel sheet having superior magnetic properties as claimed inclaim 1, wherein the amounts of Cu and P are 0.050-0.150% and0.040-0.120% respectively all in weight %.
 3. The grain orientedelectrical steel sheet having superior magnetic properties as claimed inclaim 1, wherein the value of Cu/P comes within the range of 0.50-3.0.4. A process for producing a grain oriented electrical steel sheethaving superior magnetic properties on the order of greater than about1.9 Tesla magnetic flux density. (B₁₀) and less than about 1.1 W/Kg ironor core loss (W_(17/50)), comprising the steps of:preparing a siliconsteel slab by adding 0.030-0.300% of Cu and 0.020-0.200% of P to amolten silicon steel, said silicon steel consisting essentially of0.030-0.100% of C, 2.50-4.00% of Si, 0.030-0.150% of Mn, 0.010-0.050% ofS, 0.010-0.050% of soluble Al and 0.003-0.012% of N, the balance beingFe, all in weight %; and carrying out, on said silicon steel slab, a hotrolling, a precipitation annealing, a pickling process, a cold rollingto a thickness of about 0.15-0.27 mm, a decarburizing annealing, coatingwith an annealing separator, and a high temperature annealing.